Combined hemodynamic and egm-based arrhythmia detection

ABSTRACT

A medical device and associated method for detecting arrhythmias that includes electrodes for sensing cardiac electrical signals and a hemodynamic sensor for sensing a hemodynamic signal. An episode of cardiac electrical event intervals meeting cardiac arrhythmia detection criteria is detected from the sensed electrical signals. Cardiac mechanical events and/or cardiac mechanical event intervals are measured from the hemodynamic signal and used to withhold or confirm a cardiac arrhythmia detection of the episode.

RELATED PRIORITY APPLICATION

The present application claims priority and other benefits from U.S.Provisional Patent Application Ser. No. 61/162,535, filed Mar. 23, 2009,entitled “USE OF HEMODYNAMICS AS AN ADJUNCT TO ICD TACHYCARDIA EGM-BASEDDISCRIMINATION”, incorporated herein by reference in it's entirety.

TECHNICAL FIELD

The disclosure relates generally to implantable medical devices and, inparticular, to a method and apparatus for detecting and treatingarrhythmias.

BACKGROUND

Implantable cardioverter defibrillators (ICDs) are configured toevaluate intracardiac electrogram (EGM) signals for detecting cardiacarrhythmias. Typically a fast rhythm is detected based on EGM sensedevent intervals, e.g. RR intervals. However, not all fast heart ratesmeeting arrhythmia detection criteria are lethal arrhythmias thatrequire a cardioversion or defibrillation shock. For example, a fastventricular rate may be sinus tachycardia in response to physicalexertion. In other cases, oversensing of cardiac events may result in anoverestimate of the actual heart rate, which can cause false tachycardiadetections. While it is desirable to quickly detect and treatpotentially lethal arrhythmias, such as ventricular fibrillation, it isalso desirable to avoid painful defibrillation shocks when such shocksare unnecessary. A need remains for improved methods for detectingarrhythmias and discriminating between potentially lethal arrhythmiasassociated with hemodynamic collapse and non-lethal, fast rhythms thatdo not require immediate therapy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an implantable, cardiac stimulation device embodied as animplantable cardioverter defibrillator (ICD).

FIG. 2 is a functional block diagram of the ICD shown in FIG. 1according to one embodiment.

FIG. 3 is a flow chart of a method for using hemodynamic signals in anICD for the detection of cardiac arrhythmias and in making arrhythmiatherapy decisions.

FIG. 4 is a flow chart of one method for computing and qualifyingbaseline hemodynamic measurements.

FIG. 5 shows EGM and RV pressure recordings illustrating one method forusing mechanical intervals for confirming EGM intervals.

FIG. 6 shows EGM and RV pressure recordings illustrating one method fordetecting a non-sustained arrhythmia.

FIG. 7 shows EGM and RV pressure recordings illustrating methods fordiscriminating sinus tachycardia and supraventricular tachycardia fromtrue VT/FVT or VF using hemodynamic measurements.

FIG. 8 is a state diagram illustrating transitions between a preliminarydetection state, a confirmed detection state and a withhold detectionstate during operation of an ICD.

DETAILED DESCRIPTION

In the following description, references are made to illustrativeembodiments. It is understood that other embodiments may be utilizedwithout departing from the scope of the invention. For purposes ofclarity, the same reference numbers are used in the drawings to identifysimilar elements. As used herein, the term “module” refers to anapplication specific integrated circuit (ASIC), an electronic circuit, aprocessor (shared, dedicated, or group) and memory that execute one ormore software or firmware programs, a combinational logic circuit, orother suitable components that provide the described functionality.

FIG. 1 depicts an implantable, cardiac stimulation device embodied as animplantable cardioverter defibrillator (ICD) 14, in which methodsdescribed herein may be implemented. Various embodiments of theinvention may be implemented in numerous types of implantable medicaldevices capable of sensing cardiac signals, such as pacemakers, ECGmonitors, and hemodynamic monitors. ICD 14 is provided for sensingintrinsic heart activity and delivering cardiac stimulation pulses inthe form of pacing, cardioversion or defibrillation therapy, asappropriate, to one or more heart chambers.

ICD 14 is shown in communication with a patient's heart 10 by way ofthree leads 16, 32 and 52. The heart 10 is shown in a partially cut-awayview illustrating the upper heart chambers, the right atrium (RA) andleft atrium (LA), and the lower heart chambers, the right ventricle (RV)and left ventricle (LV), and the coronary sinus (CS) in the right atriumleading into the great cardiac vein 48, which branches to form inferiorcardiac veins. Leads 16, 32 and 52 connect ICD 14 with the RA, the RVand the LV, respectively. Each lead has at least one electricalconductor and pace/sense electrode. A remote indifferent can electrodeis formed as part of the outer surface of the ICD housing 20. Thepace/sense electrodes and the remote indifferent can electrode can beselectively employed to provide a number of unipolar and bipolarpace/sense electrode combinations for pacing and sensing functions.

RA lead 16 is passed through a vein into the RA chamber and may beattached at its distal end to the RA wall using a fixation member 17. RAlead 16 is formed with a connector 13 fitting into a connector bore ofICD connector block 12 for electrically coupling RA tip electrode 19 andRA ring electrode 21 to ICD circuitry housed within housing 20 viainsulated conductors extending within lead body 15. RA tip electrode 19and RA ring electrode 21 may be used in a bipolar fashion, or in aunipolar fashion with ICD housing 20, for achieving RA stimulation andsensing of RA EGM signals. RA lead 16 is also provided with a coilelectrode 18 that may be used for delivering high voltagecardioversion/defibrillation pulses to heart 10 in response to thedetection of tachycardia or fibrillation.

RV lead 32 is passed through the RA into the RV where its distal end,carrying RV tip electrode 40 and RV ring electrode 38 provided forstimulation in the RV and sensing of RV EGM signals, is fixed in placein the RV apex by a distal fixation member 41. RV lead 32 also carries ahigh-voltage coil electrode 60 for use in cardioverting anddefibrillating heart 10. RV lead 32 is formed with a connector 34fitting into a corresponding connector bore of ICD connector block 12.Connector 34 is coupled to electrically insulated conductors within leadbody 36 and connected with distal tip electrode 40, ring electrode 38and coil electrode 60.

RV lead 32 additionally carries a pressure sensor 70 for sensing RVpressure signals. Pressure sensor 70 is coupled to ICD 14 via aconductor extending through lead body 36. In alternative embodiments,any of leads 16, 32 and 52 may carry a physiological sensor producing asignal responsive to the hemodynamic function of the heart 10. As willbe described in detail herein, a hemodynamic signal is used inconjunction with a sensed EGM signal for detecting and discriminatingarrhythmias.

Coronary sinus lead 52 is passed through the RA, into the CS and furtherinto a cardiac vein 48 to extend the distal LV tip electrode 50 and ringelectrode 62 alongside the LV chamber to achieve LV stimulation andsensing of LV EGM signals. The LV CS lead 52 is coupled at the proximalend connector 54 into a bore of ICD connector block 12 to provideelectrical coupling of conductors extending from electrodes 50 and 62within lead body 56 to ICD internal circuitry. In some embodiments, LVCS lead 52 could bear a proximal LA pace/sense electrode 51 positionedalong CS lead body 56 such that it is disposed proximate the LA for usein stimulating the LA and/or sensing LA EGM signals.

In addition to the lead-mounted electrodes, ICD 14 may include one ormore subcutaneous cardiac sensing electrodes (not shown) formed asuninsulated portions of the ICD housing 20 or included in the connectorblock 12. While a particular ICD system with associated leads andelectrodes is illustrated in FIG. 1, numerous implantable cardiacmonitoring, pacemaker and ICD system configurations are possible, whichmay include one or more leads deployed in transvenous, subcutaneous, orepicardial locations. The lead and electrode arrangements will depend onthe particular application. Methods described herein which combinedhemodynamic and EGM signal analysis in arrhythmia detection algorithmsmay also be implemented in subcutaneous cardiac monitor, pacemaker orICD systems in which electrodes are formed as a part of the devicehousing and/or carried by subcutaneous leads.

ICD 14 is shown as a multi-chamber device capable of sensing andstimulation in three or all four heart chambers. It is understood thatICD 14 may be modified to operate as a single chamber device or dualchamber device. In the illustrative embodiments, described herein,methods for arrhythmia detection are described in which ventricular EGMand pressure signals are sensed and used for detecting arrhythmias andmaking therapy delivery decisions. It is contemplated that the methodsdescribed, however, may be used in dual chamber, triple chamber or allfour chamber applications in which both ventricular and atrial EGMsignals are available.

FIG. 2 is a functional block diagram of the ICD 14 shown in FIG. 1according to one embodiment. ICD 14 generally includes timing andcontrol circuitry 152 and an operating system that may employmicroprocessor 154 or a digital state machine for timing sensing andtherapy delivery functions in accordance with a programmed operatingmode. Microprocessor 154 and associated memory 156 are coupled to thevarious components of ICD 14 via a data/address bus 155. ICD 14 includestherapy delivery module 150 for delivering electrical stimulationtherapies, such as cardiac pacing therapies and arrhythmia therapiesincluding cardioversion/defibrillation shocks and anti-tachycardiapacing (ATP), under the control of timing and control 152. Therapydelivery module 150 is typically coupled to two or more electrodes 168via an optional switch matrix 158. Switch matrix 158 is used forselecting which electrodes and corresponding polarities are used fordelivering electrical stimulation pulses.

Cardiac electrical signals are sensed for use in determining when anelectrical stimulation therapy is needed and in controlling astimulation mode and the timing of stimulation pulses. Electrodes 168used for sensing are coupled to signal processing circuitry 160. Signalprocessor 160 includes sense amplifiers and may include other signalconditioning circuitry and an analog-to-digital converter. Electricalsignals may then be used by microprocessor 154 or other controlcircuitry for detecting physiological events, such as detecting anddiscriminating cardiac arrhythmias. Signal processing circuitry 160 mayinclude event detection circuitry generally corresponding to R-wavedetection circuitry as disclosed in U.S. Pat. No. 5,117,824 (Keimel, etal.), hereby incorporated herein by reference in its entirety.

Arrhythmia detection algorithms may be implemented for detectingtachycardia and fibrillation and for discriminating between ventriculararrhythmias and supraventricular arrhythmias (SVTs). For example,ventricular tachycardia (VT), fast ventricular tachycardia (FVT) andventricular fibrillation (VF) may be detected using sensed EGM signals.Ventricular event intervals (R-R intervals) sensed from the EGM signalsare commonly used for detecting ventricular arrhythmias. Additionalinformation obtained such as R-wave morphology, slew rate, other eventintervals (P-R intervals) or other sensor signal information may be usedin detecting, confirming or discriminating an arrhythmia. Reference ismade to U.S. Pat. No. 5,354,316 (Keimel); U.S. Pat. No. 5,545,186 (Olsonet al.); and U.S. Pat. No. 6,393,316 (Gillberg et al.) for examples ofarrhythmia detection and discrimination using EGM signals, all of whichpatents are incorporated herein by reference in their entirety.

In one detection scheme, programmable detection interval rangesdesignate the range of sensed event intervals indicative of atachycardia and may be defined separately for detecting VT, FVT and VF.Sensed event intervals falling into defined detection interval rangesare counted to provide a count of tachycardia or fibrillation intervals.A programmable number of intervals to detect (NID) defines the number oftachycardia/fibrillation intervals occurring consecutively or out of agiven number of preceding event intervals that are required to detecttachycardia or fibrillation. A separately programmed NID may be definedfor detecting VT, FVT, and VF. In addition to the interval ranges andNID criteria, rapid onset criterion and rate stability criterion mayalso be defined for use in arrhythmia detection schemes. Furthermore, acombined count of tachycardia and fibrillation intervals may be comparedto a combined count threshold and, according to predefined criteria,used in detecting fibrillation or slow or fast tachycardia.

In addition to event interval information, the morphology of the EGMsignal may be used in discriminating heart rhythms, for example asdescribed in the above-incorporated '316 Gillberg patent. According toone embodiment of the invention, digitized EGM signals are provided tomicroprocessor 154 for waveform analysis according to an implementedmorphology or template matching algorithm. Morphology analysis may beused in conjunction with event interval analysis to improve thesensitivity and specificity of arrhythmia detection methods.

ICD 14 is additionally coupled to one or more physiological sensors 170.Physiological sensors 170 include at least one sensor responsive tocardiac hemodynamic function. In one embodiment, sensors 170 include apressure sensor adapted for placement within a ventricle of the heart asshown in FIG. 1 for providing an intraventricular pressure signal. Inother embodiments, sensors 170 may include a motion sensor such as anaccelerometer, a flow sensor, blood chemistry sensors such as an oxygensaturation sensor, activity sensors, an acoustical sensor, or otherphysiological sensors. Physiological sensors may be carried by any leadextending from ICD 14, incorporated in or on the ICD housing or may beembodied as leadless sensors implanted in the body and in telemetriccommunication with the ICD or another device.

As will be described in detail herein, a hemodynamic signal is acquiredfrom physiological sensors 170 for use in arrhythmia detection andtherapy delivery decisions. As used herein, the term “hemodynamicsignal” refers generally to a signal measuring effects of the mechanicalpumping function of the heart. A hemodynamic signal may be, but notnecessarily limited to, a motion signal, a pressure signal, a flowsignal, or an acoustical signal. Accordingly, hemodynamic sensorsinclude sensors generating a signal corresponding to heart or vesselwall motion, arterial or intracardiac blood pressure, blood flow, orheart sounds. The hemodynamic signal is used to verify an arrhythmiaepisode detection made using cardiac electrical signals and for makingappropriate therapy delivery decisions based on the hemodynamic statusof the patient at the time of an arrhythmia episode detection.

Signals from sensors 170 are received by a sensor interface 162 whichprovides sensor signals to signal processing circuitry 160. Sensorsignals are used by microprocessor 154 for detecting physiologicalevents or conditions. For example, ICD 14 may monitor heart wall motion,blood pressure, blood chemistry, respiration, or patient activity.Monitored signals may be used for sensing the need for delivering oradjusting a therapy under control of the operating system. Monitoredsensor signals may be analyzed to obtain diagnostic or prognostic datastored by ICD and made available to a clinician.

The ICD operating system includes associated memory 156 for storing avariety of programmed-in operating mode and parameter values that areused by microprocessor 154. Algorithms for detecting arrhythmias,delivering arrhythmia therapy, and associated parameter values may bestored in memory 156. The memory 156 may also be used for storing datacompiled from sensed EGM and physiological signals and/or relating todevice operating history for telemetry out on receipt of a retrieval orinterrogation instruction.

In response to an arrhythmia detection, a programmed arrhythmia therapyis delivered by therapy delivery module 150 under the control of timingand control 152. A description of high-voltage output circuitry andcontrol of high-voltage shock pulse delivery is provided, for example,in the above-incorporated '186 Olson patent. Typically, a tiered menu ofarrhythmia therapies are programmed into the device ahead of time by thephysician and stored in memory 156. For example, on initial detection ofVT, an anti-tachycardia pacing therapy may be delivered. On redetectionof tachycardia, a more aggressive anti-tachycardia pacing therapy may bescheduled. If repeated attempts at anti-tachycardia pacing therapiesfail, a high voltage cardioversion pulse may be selected to moreaggressively treat the detected arrhythmia. Therapies for tachycardiatermination may vary with the rate of the detected tachycardia, with thetherapies increasing in aggressiveness as the rate of the detectedtachycardia increases. For example, fewer attempts at anti-tachycardiapacing may be undertaken prior to delivery of cardioversion pulses ifthe rate of the detected tachycardia is above a preset threshold.

In the event that fibrillation is identified, which may be atrialfibrillation or ventricular fibrillation, high frequency burststimulation may be employed as the initial attempted therapy. Subsequenttherapies may include high voltage defibrillation pulses. Thedefibrillation pulse energy may be increased if an initial shock pulsefails to terminate fibrillation. As will be described herein,hemodynamic signal information may be used to modify the progression ofa programmed menu of tiered therapies.

ICD 14 further includes telemetry circuitry 164 and antenna 165.Programming commands or data are transmitted during uplink or downlinktelemetry between ICD telemetry circuitry 164 and external telemetrycircuitry included in a programmer or monitoring unit.

FIG. 3 is a flow chart 200 of a method for using hemodynamic signals inan ICD for the detection of cardiac arrhythmias and in making arrhythmiatherapy decisions. Flow chart 200 is intended to illustrate thefunctional operation of the device, and should not be construed asreflective of a specific form of software or hardware necessary topractice the methods described. It is believed that the particular formof software, firmware or hardware will be determined primarily by theparticular system architecture employed in the device and by theparticular detection and therapy delivery methodologies employed by thedevice. Providing software, firmware or hardware to accomplish thedescribed functionality in the context of any modern ICD, given thedisclosure herein, is within the abilities of one of skill in the art.

Methods described in conjunction with flow charts presented herein maybe implemented in a computer-readable medium that includes instructionsfor causing a programmable processor to carry out the methods described.A “computer-readable medium” includes but is not limited to any volatileor non-volatile media, such as a RAM, ROM, CD-ROM, NVRAM, EEPROM, flashmemory, and the like. The instructions may be implemented as one or moresoftware modules, which may be executed by themselves or in combinationwith other software.

At block 202, baseline hemodynamic measurements are acquired, stored andevaluated for use in arrhythmia detection. In one embodiment, at leasttwo baseline measurements are acquired, a long-term (LT) baselinehemodynamic measurement and a sinus tachycardia (ST) baselinemeasurement. The LT and ST baseline measurements will be compared toqualification criteria to determine if the hemodynamic baselinemeasurements are useful and reliable for arrhythmia detection andtherapy delivery decisions. Computation and qualification of baselinemeasurements will be described in greater detail below in conjunctionwith FIG. 4.

At block 204, cardiac EGM signals and a hemodynamic signal are sensedfor rhythm monitoring. In the illustrative embodiments described herein,the hemodynamic signal relied upon for arrhythmia detection and therapydelivery decision-making is an RV pressure signal. However, it isrecognized that the methods described herein may be adapted to use otherblood pressure signals, flow signals, motion signals, heart soundsignals or any other available hemodynamic signal responsive to cardiacmechanical function.

At block 206, electrical event interval counters are updated as cardiacelectrical events, i.e., R-waves (and P-waves in dual chamber devices),are sensed. Electrical event interval counters track the number ofintervals meeting arrhythmia detection criteria. More specifically, in asingle chamber ventricular ICD, for example, R-waves are sensed and RRintervals are measured. The RR intervals are compared to stored intervalranges for arrhythmia detection zones. As described previously,detection interval ranges are defined for different rate zones, e.g.,VT, fast VT and VF rate zones. A number of intervals to detect (NID) areadditionally defined for each rate zone. The detection interval rangeand the NID for each rate zone are typically programmable. As RRintervals falling in an arrhythmia detection interval range aredetected, a corresponding interval counter (VT, FVT, VF or combinedcount) is updated at block 206.

If any of the interval counters reach the NID criteria, a preliminary,i.e., not yet confirmed, arrhythmia detection is made at block 210 basedonly on the EGM interval criteria. When a preliminary arrhythmiadetection is made, a hemodynamic analysis timer is optionally set atblock 212. During the operation of method 200, one or more timers may beset to control the amount of time allowed for an arrhythmia detection tobe withheld based on hemodynamic signal analysis. A hemodynamic analysistimer may be set to different intervals depending on the detected heartrate. For example, a hemodynamic analysis timer may be set to a shorterinterval when a preliminary VF detection is made at block 206, and thetimer may be set to a relatively longer interval when VT ispreliminarily detected.

Furthermore, a hemodynamic analysis timer may be set to differentintervals depending on whether the detection made at block 206 is theinitial, i.e., first confirmed, detection of an arrhythmia episode orredetection of the arrhythmia episode. Accordingly, a hemodynamicanalysis timer may be set to at least 4 different time intervalscorresponding to at least four different conditions occurring at block210 based on cycle length conditions (for example VT detection range orVF detection range) and initial detection and redetection conditions. Inan alternative embodiment, separate timers may be set for controllingthe amount of time allowed to withhold an arrhythmia detection based onthe hemodynamic analyses performed at each of the respective blocks 216,218, 222, and 224. It is recognized that a hemodynamic analysis timerlimiting the time an arrhythmia detection is withheld due to hemodynamicsignal information is optional and when implemented may be programmed tobe disabled or “off”.

At block 214, mechanical event interval and mechanical eventmeasurements are acquired or updated for use in hemodynamic analysesperformed at blocks 216 through 224. Mechanical event intervals andhemodynamic measurements are obtained from the sensed hemodynamicsignal.

The hemodynamic signal may be recorded without performing signalanalysis until a preliminary arrhythmia detection is made.Alternatively, hemodynamic event interval and event measurements can bemade from a hemodynamic signal on a continuous basis so that hemodynamicdata is immediately available upon preliminary arrhythmia detection.Such data may then be evaluated retrospectively when a preliminaryarrhythmia detection is made. Various measurements of hemodynamic eventsand event intervals are made at block 214 for use in comparing tobaseline measurements and to the detected electrical event intervals,respectively, as will be further described herein.

At block 216, mechanical event intervals are used to confirm theelectrical event intervals measured from the EGM signal leading to thepreliminary arrhythmia detection made at block 210. One method for usingmechanical event intervals to confirm the presence of a fast electricalevent rate is described below in conjunction with FIG. 5.

At block 218, mechanical event measurements are used to detect a breakin the arrhythmia episode. In other words, measurements of mechanicalevents are used to detect a non-sustained arrhythmia. Good hemodynamicfunction associated with an RR interval that is longer than a maximumarrhythmia detection interval and occuring during the preliminarilydetected arrhythmia episode is evidence of a non-sustained arrhythmia.One method for detecting a break in the arrhythmia episode is describedbelow in conjunction with FIG. 6.

At block 220, mechanical event measurements are used to discriminatebetween a potentially lethal arrhythmia, associated with hemodynamiccollapse, and non-lethal arrhythmias, in which hemodynamic function isexpected to be adequate to sustain consciousness. If hemodynamiccollapse is detected, the preliminary arrhythmia detection is confirmedand programmed therapies are immediately delivered according to a menuof tiered therapies at block 224. Alternatively, a programmed menu oftiered therapy may be overridden in response to detecting hemodynamiccollapse and a more aggressive therapy, e.g., a high voltage shocktherapy, may be immediately delivered.

If hemodynamic collapse is not detected at block 220, but hemodynamicdata confirms the EGM-based arrhythmia detection up to this point (EGMintervals confirmed at block 216 and no rhythm break detected at block218), additional hemodynamic measurement rules are applied for use inmaking an appropriate therapy delivery decision. At block 222,hemodynamic criteria are applied to the mechanical event measurementsfor discriminating between pathologic VT and sinus tachycardia.

If the preliminary detected episode is determined to be sinustachycardia based on the hemodynamic criteria, arrhythmia detection iswithheld at block 232. Therapy delivery is not initiated. Method 200returns to block 206 to continue monitoring the electrical eventintervals and update EGM interval counters. Mechanical event intervalsand measurements will be updated upon redetection of the arrhythmiaepisode. Mechanical event data may be logged in memory with otherepisode data such that it is available for later review by a clinician.

At block 224, hemodynamic criteria are applied to the mechanical eventmeasurements to discriminate between ventricular and supraventriculararrhythmias. If the detected episode is determined to be SVT based onhemodynamic criteria being satisfied, the preliminary ventriculararrhythmia detection is withheld at block 232, and ventricular therapydelivery is not initiated. Method 200 returns to block 204 to continuemonitoring electrical event intervals and update the mechanical eventmeasurements as needed.

If the hemodynamic measurements do not meet sinus tachycardia or SVTrules applied at blocks 222 and 224, additional analysis of the EGMsignals may be performed at blocks 226 and 228 to discriminate orconfirm the detected arrhythmia episode. For example, at block 226,measured RR intervals may be compared to SVT interval criteria fordiscriminating between VT and SVT. In one embodiment, if a median RRinterval is determined to be shorter than an SVT RR interval limit atblock 226, the preliminary VT, FVT or VF initially made at block 206 isconfirmed. Method 200 advances to block 230 to deliver arrhythmiatherapy according to a programmed menu of tiered therapy.

If SVT criteria applied to the RR interval measurements at block 226 aremet, morphological analysis of the EGM signal is performed at block 228.For example, wavelet analysis may be performed as generally described inthe above-referenced Gillberg patent. Based on the wavelet or othermorphological analysis of the EGM signal waveform, block 228, thepreliminary ventricular arrhythmia detection made at block 206 may beeither confirmed or withheld. If confirmed, the appropriate VT, FVT orVF therapy is delivered at block 230 according to programmed therapies.If withheld, method 200 returns to block 206 to continue updatingelectrical event interval counters and mechanical event intervals andmeasurements.

If the preliminary arrhythmia episode is redetected at block 210 afterwithholding an arrhythmia detection due to any of the hemodynamicanalyses at blocks 216, 218, 222, and 224, any or all of the hemodynamicanalysis performed at those blocks may be disabled. For example, if theelectrical event intervals are confirmed at block 216 based on measuredmechanical event intervals following a preliminary arrhythmia detection,the analysis at block 216 may be disabled following an EGMinterval-based redetection of the same arrhythmia episode. In otherwords, once the electrical event intervals are confirmed to not beoverestimating the heart, for example due to oversensing, the mechanicalevent intervals are not used again to verify the electrical eventintervals. Likewise, hemodynamic analysis performed at block 220 may bedisabled following a preliminary arrhythmia detection and enabled todetect hemodynamic collapse following an EGM interval-based redetectionof the same episode. The hemodynamic collapse detection at block 220 mayalso be applied only when the preliminary detection or redetection is aFVT or VF and not applied for a preliminary detection or redetection ofa VT.

If the hemodynamic analysis timer expires (block 240), method 200 mayimmediately advance to perform additional EGM signal analysis at block226 and/or block 228 and skip additional hemodynamic analysis. In oneembodiment, if the timer expires at block 240, the hemodynamic analysisis abandoned and wavelet analysis is immediately performed at block 228.If the wavelet analysis results in a withhold detection decision atblock 232, then the hemodynamic analysis timer may be reset at block214.

The hemodynamic analysis timer may also be reset if EGM interval(s) falloutside the arrhythmia detection zones or NID criteria are no longerbeing met. In one embodiment, the hemodynamic analysis timer is notreset at block 212 when the withhold detection decision at block 232 isarrived at through any hemodynamic analysis block 216, 218, 222, or 224.In this way, a prolonged withholding of arrhythmia detection and therapydelivery does not occur based on hemodynamic analysis alone. EGManalysis, for example wavelet analysis at block 228, is performed toensure that prolonged detection and therapy withholding decisions aresupported by both hemodynamic and EGM analysis and not hemodynamicanalysis alone.

In other embodiments, interval-based, hemodynamic-based, andmorphology-based measurements may each receive a level of confidence inconfirming the presence of an arrhythmia. The arrhythmia detection maybe made based on the confidence of each parameter measured. A singleparameter with high confidence or the combined confidence of multipleparameters may lead to an arrhythmia detection. If no single or combinedconfidence lead to an arrhythmia detection, a preliminary detectionbased on a single parameter may be withheld until at least one of theother parameters worsens to reach a detection confirmation criteria orsome additional amount of time has passed with no changes in theconfidences of the single or other parameters. Thus, both hierarchicaland non-hierarchical approaches to detecting an arrhythmia usingmultiple parameter analyses of both an EGM signal and a hemodynamicsignal may be used.

FIG. 4 is a flow chart of one method 300 for computing and qualifyingbaseline hemodynamic measurements at block 202 of method 200. Asdiscussed previously, both long-term and sinus tachycardia baselinemeasurements are computed in one embodiment. The LT baselinemeasurements represent baseline hemodynamic function over a relativelylong period of time, e.g., to encompass a range of the patient's typicalactivities of daily living. The ST baseline measurements correspond to aperiod of increased metabolic activity when the patient is in normalsinus rhythm, but at an elevated heart rate.

At block 302, an RV pressure signal is sensed continuously for computinginitial baseline measurements and regularly updating the baselinemeasurements upon expiration of an update interval at block 304. Method300 remains at block 304 storing hemodynamic signal data until theinitial measurement period or an update interval expires.

In one embodiment, the LT baseline measurements are computed as 24-houraverage values of a hemodynamic measurement. In the specific example ofFIG. 4, RV pressure is continuously monitored and a 24-hour averagepulse pressure is initially computed at block 306 then updated fromstored data every 12 hours (or at another update interval within, at, orgreater than the 24-hour period). In addition to a 24-hour average pulsepressure, a 24-hour average positive maximum dP/dt is computed at block308. These two measurements are used as LT baseline measurements forcomparison to hemodynamic measurements in response to an EGMinterval-based preliminary arrhythmia detection for confirming ordiscriminating the arrhythmia. One detailed method for measuring pulsepressures and dP/dt on a beat-by-beat basis for use in computingbaseline measurements is described below in conjunction with FIG. 5.

Other baseline measurements that may be derived from a pressure signalinclude a peak pressure or the area of a portion of the pressurewaveform. It is recognized that if other types of hemodynamic signalsare available, appropriate baseline measurements may be computed whichmay include a signal average, average peak-to-peak differences orabsolute peak amplitudes, a peak rate of change, an integral or othermeasurements of the mechanical hemodynamic events.

The LT baseline measurements obtained over a relatively long period oftime, for example at least one hour or one day, provide more relevantbaseline measures than hemodynamic measurements obtained in the secondsor minutes just prior to a preliminary arrhythmia episode detection.Since hemodynamic function may already be altered due to changes inautonomic tone or other physiological changes leading up to anarrhythmia, hemodynamic measurements taken in the minutes or secondsbefore an arrhythmia detection may not accurately represent thepatient's normal hemodynamic baseline function. The LT baselinemeasurements averaged over several hours and a range of patient activityare less susceptible to noise and effects of beat-to-beat variability,which can be observed in hemodynamic signals such as a right ventricularpressure signal.

At block 310, an increased metabolic demand is detected. In variousembodiments, an increased metabolic demand may be detected as aconfirmed episode of sinus tachycardia, an elevated heart rate that isabove a resting rate but not yet meeting a tachycardia rate, elevatedactivity detected using an activity sensor, increased respiration, orany combination thereof. In one embodiment, increased metabolic demandis detected as a heart rate that is in the range of approximately 90 toapproximately 110 beats per minute. In other embodiments, increaseddemand may be defined as a percentage or fixed interval above apatient's measured resting heart rate.

ST baseline measurements of average pulse pressure and average maximumdP/dt are computed at blocks 314 and 316 from selected samplingintervals of the monitored RV pressure signal corresponding to aninterval of time in which the average heart rate is at a predefinedelevated level (or when another measure of increased metabolic demand isdetected). The sampling intervals may be identified as time intervalsgreater than a predefined minimum time interval, e.g. at least 30seconds, during which the average heart rate is within a predeterminedrange, such as 90 to 110 beats per minute. In general, the RV pressuresignal data is sampled during intervals in which the patient is believedto be experiencing sinus rhythm under increased metabolic demandconditions such that an expected increased hemodynamic response can bemeasured. The hemodynamic function measured under increased metabolicdemand can be used to discriminate sinus tachycardia from pathologicalforms of tachycardia.

The ST baseline may be updated at the same update interval, e.g. every12 hours, as the LT baseline, or another update interval, or each time asampling interval is identified. The sampling intervals used forcomputing the ST baseline measurements may be selected from a longerperiod of time than the update interval. For example, all samplingintervals meeting the sampling criteria occurring during the past 24hours may be used to update the ST baseline measurements every 12 hours.At decision block 310, method 300 determines if a minimum number of datapoints are available from sampling intervals meeting the increasedmetabolic demand detection criteria during the past 24 hour period orother defined sampling period. If not, the ST baseline measurements arenot updated.

An ST baseline measurement may be considered valid for use in anarrhythmia detection algorithm for a predefined validity period, forexample for 2 days, 3 days or longer. If the ST baseline measurement isnot updated before the validity period expires, as determined at block312, the ST baseline measurements are considered expired or invalid foruse in arrhythmia detection and therapy delivery decision-making atblock 322. As will be described below, the ST baseline measurements areused for discriminating between VT, SVT and sinus tachycardia. As such,these hemodynamic-based discrimination steps in method 200 may bedisabled when an ST baseline measurement is expired. The LT baselinemeasurements, if qualified, may still be used for detecting hemodynamiccollapse and confirming a preliminary arrhythmia detection and may beused in arrhythmia discrimination methods.

In alternative embodiments, an ST baseline may be set as a function ofthe LT baseline, for example a multiple or percentage greater than theLT baseline. The ST baseline is used to discriminate sinus tachycardiafrom a pathologic tachycardia. The ventricular pulse pressure andmaximum dP/dt during sinus tachycardia are expected to exceed LTbaseline measurements of these variables in a patient having enoughcardiac reserve to produce a hemodynamic response to increased metabolicdemand.

At blocks 318 and 320, comparative analysis of the measured baselines isperformed to determine if the baseline measurements qualify for use inarrhythmia detection and therapy delivery decision-making. It isrecognized that, during the computation of the baseline measurements atblocks 306, 308, 314 and 316, outlier or artifact values of the measuredhemodynamic signal may be discarded when determining average or medianvalues of the baseline measurements. Numerous methods for artifactrejection may be employed. However, once a baseline measurement isobtained, the magnitude of the hemodynamic measurement is examined todetermine if reliable distinction between normal hemodynamic functionand hemodynamic collapse is possible.

For example, if the RV pressure pulse is weak, the signal-to-noise ratiomay be poor and the accuracy and reliability of hemodynamic-basedalgorithms for arrhythmia detection may be limited. Hemodynamic collapsemay be difficult to distinguish from normal hemodynamic function if thepulse pressure signal under normal conditions is already weak.

In one embodiment, if an average pulse pressure baseline is less than apredetermined threshold, e.g. if the LT average pulse pressure is lessthan approximately 20 mmHg, as determined at block 318, the baselinemeasurements are disqualified for use in arrhythmia detection andtherapy delivery decision-making purposes at block 326. Hemodynamicarrhythmia detection algorithms, such as method 200 of FIG. 3, will notbe enabled at block 328. Baseline measurements may be repeated at alater time in an attempt to obtain measurements meeting thequalification criteria.

The RV pressure pulse and maximum dP/dt are expected to be higher duringsinus tachycardia than during rest. A measurable difference between theLT baseline measurements and the ST baseline measurements thus enablesdiscrimination between sinus tachycardia, supraventricular tachycardias,and true VT/FVT or VF. If a patient is lacking cardiac reserve, i.e. thehemodynamic response during exercise is not significantly different thanduring rest, LT and ST baseline measurements will not be significantlydifferent. If valid ST baseline measurements are available (i.e., notexpired as determined at block 312) the valid ST baselines are comparedto the LT baselines at block 320. If the ST baselines are not greaterthan the LT baseline measurements by at least a predetermined margin oferror, as determined at block 320, the ST baseline measurements may bedisqualified for use in arrhythmia detection and therapy deliverydecisions at block 324. Since the LT baseline measurements werequalified at block 318, the hemodynamic detection algorithm is stillenabled at block 328. Only portions of the hemodynamic-based algorithmthat rely on the ST baseline measurements will be disabled. In someembodiments, a multiple of the LT baseline measurements may besubstituted for the ST baseline measurements when a ST baseline isdetermined to be invalid or disqualified.

If both the LT and ST baseline measurements meet the qualificationcriteria at blocks 318 and 320, indicating the pressure signal providesacceptable signal strength and the LT and ST baseline measurements aredistinguishable from each other, discrimination between hemodynamiccollapse, normal sinus rhythm and sinus tachycardia using hemodynamicanalysis is possible. An algorithm relying on hemodynamic analysis forarrhythmia detection and therapy delivery decisions is fully enabled atblock 328.

It is contemplated that numerous methods may be used for computingbaseline measurements. Various embodiments may use different numbers ofdata points for computing average, median or other statistical values ofhemodynamic measurements over predetermined sampling periods. Likewisenumerous artifact rejection and signal quality control measures may beimplemented to ensure that baseline hemodynamic measurements arereliable.

FIG. 5 shows EGM and RV pressure recordings illustrating one method forusing mechanical event intervals for confirming EGM event intervals atblock 216 of method 200. RR intervals are measured from a ventricularEGM signal 402. In the example shown, a preliminary FVT detection ismade at 406 when a fast VT interval counter reaches a programmed NID.Upon making the preliminary arrhythmia detection based on the electricalevent intervals, mechanical event intervals are evaluated from the RVpressure signal 404 to confirm the detected electrical rate.

Mechanical event intervals are identified independent of the sensedelectrical intervals. In one embodiment, mechanical event intervals 414are measured for a predetermined time interval 430 immediately prior tothe preliminary arrhythmia detection 406. Time interval 430 may bebetween 2 and 12 seconds, e.g. approximately 3 seconds, or anotherpredetermined interval.

An onset 410 of a mechanical event interval 414 is identified accordingto a threshold crossing or other criteria applied to the RV pressuresignal 404 for separating RV pressure events, i.e. each pressure pulsewaveform. In one embodiment, the onset 410 is identified as the firstsample point of three consecutive points sampled at a rate of 64 Hz andhaving a sample-to-sample difference of approximately 2 mmHg that ispositive-going. It is recognized that other criteria may be set for therequired the number of sample points and point-to-point magnitudedifference and such criteria will depend in part on the sampling rate.An onset 410 may be detected only if a previous onset has not beendetected for a predetermined time interval or number of preceding samplepoints. The mechanical event interval 414 is then measured as the timeinterval between two consecutive mechanical event onsets 410 and 412.

In alternative embodiments, an onset 410 of a mechanical event intervalcan be identified by filtering the pressure signal or differentiatingthe signal to obtain a dP/dt waveform. A crossing of a dP/dt thresholdby the dP/dt waveform may define onset 410. Using a relatively low dP/dtthreshold, e.g. approximately 200 mmHg/s or less, for exampleapproximately 100 mmHg/s, allows even relatively small pressure pulseevents to be identified making accurate mechanical event intervals to bereliably measured.

The mechanical event intervals 414 are measured over the analysis timeperiod 430 and an average or median event interval is computed. Theaverage mechanical event interval is then compared to a measure of theelectrical event intervals obtained from the EGM signal. For example,the average, median, or other statistical measure of a predeterminednumber of consecutive RR intervals preceding the preliminary arrhythmiadetection 406 may be compared to the average mechanical event interval.

In one embodiment, the average mechanical event interval measured over a3 second time interval 430 is compared to the second smallest RRinterval measured in the eight RR intervals preceding the preliminaryarrhythmia detection. If the average mechanical event interval isapproximately two times longer than the electrical event intervalmeasurement, there is evidence of T-wave oversensing of the EGM signal.Each cardiac cycle is counted twice on the EGM signal when both theR-wave and T-wave are each sensed on each cardiac cycle, approximatelydoubling the actual heart rate.

Referring to method 200 of FIG. 3, in one embodiment the electricalevent intervals are confirmed as valid arrhythmia detection intervals atblock 216 if the mechanical event interval measurement meets a signalquality threshold and is less than approximately 1.8 times theelectrical event interval measurement. This confirmation reduces theoccurrence of false arrhythmia detection caused by T-wave oversensing.If the mechanical event interval measurement is greater thanapproximately 1.8 times the electrical event interval measurement,arrhythmia detection is withheld at block 232. The withholding of apreliminary arrhythmia detection made based on EGM intervals results inthe withholding of device-delivered arrhythmia therapies. Oversensing ofthe EGM signal may be causing a high rate to be falsely detected. Method200 returns to block 206 to acquire more data before confirming anarrhythmia detection. The preliminary detection itself is not in effectcancelled in that additional analysis performed of the EGM signal and/orthe hemodynamic signal may result in redetection and/or confirmation ofthe arrhythmia episode. The effect of withholding a final detection orconfirmation of the preliminary detection at block 232 is to withholdtherapy delivery to allow more data to be analyzed to confirm the rhythmand select the most appropriate therapy.

Referring again to FIG. 5, the pulse pressure may be determined for amechanical event as the difference between a maximum pressure 416 and aminimum pressure 418. The maximum pressure 416 is identified as themaximum sample point occurring during the event interval 414. Theminimum pressure 418 may be identified as the minimum sample pointduring a predetermined onset sampling interval, e.g. approximately 100ms, preceding the onset 410. Alternatively, the minimum pressure may beidentified as the minimum sample point during the event interval 414 orduring the preceding event interval.

A pulse pressure measurement may be used to verify that the RV pressuresignal strength is high enough to allow reliable mechanical eventinterval measurements. As can be seen in FIG. 5, two pressure pulses aremissed due to their very low amplitude. If the average pulse pressureduring interval 430 is less than a predetermined minimum signal qualitythreshold, e.g. less than approximately 20 mmHg, the mechanical eventintervals may be rejected for use in verifying the electrical eventintervals. Mechanical event interval analysis will not be used towithhold an EGM-based arrhythmia detection.

Similarly, a minimum number of mechanical event intervals identifiedduring the interval 430 may be required before using the mechanicalevent intervals for verifying electrical event intervals. If the pulsepressure signals are so low that less than a required number ofmechanical event intervals are identified, for example less than twointervals, mechanical event interval analysis is not used to withholdthe EGM-based arrhythmia detection. In some embodiments, only a singlemechanical event interval identified during interval 430 may berequired.

The methods shown in FIG. 5 for identifying mechanical event intervals414 using a detected onset 410 to allow measurement of pulse pressure asthe difference between a maximum 414 and minimum 418 on a beat-by-beatbasis may also be used during the computation of the LT and ST averagebaseline measurements described above. A maximum positive dP/dt may alsobe measured for each mechanical event, for example a maximum positivedP/dt occurring after an identified onset 410. The maximum positivedP/dt values are used for computing LT and ST baseline dP/dtmeasurements.

In alternative embodiments, sensed electrical events (e.g. R-waves) andpaced events may be used as fiducial points for setting a time window inwhich a mechanical event measurement is taken, such as maximum positivedP/dt or pulse pressure. In one embodiment a measurement window is setupon an EGM sensed or paced event. The occurrence of a mechanical eventwithin that measurement window, or within a separately definedmechanical event window, is verified by identifying a positive slope ofthe hemodynamic signal followed by a negative slope of the hemodynamicsignal. For example, if 3 consecutive 64 Hz sample points have apositive-going difference of at least 2 mmHg, then a pressure pulseupslope is identified. An upslope may alternatively be identified as apositive maximum dP/dt. A pressure pulse downslope is identified bysearching, beginning after a maximum signal point following the upslope,for at least two consecutive 64 Hz sample points having a negative-goingdifference of at least 2 mmHg. When the upslope and downslope points areidentified and occur a predetermined time range apart, the occurrence ofthe pressure pulse is confirmed. Mechanical event measurements, such asdP/dt and pulse pressure may then be identified within the EGM-basedmeasurement time window.

In one embodiment, the upslope and downslope points must be identifiedwithin a mechanical event window beginning approximately 100 ms after asense or paced event and extending until the next sense or paced eventor 600 ms, whichever comes first. In addition, in the illustrativeembodiment, the upslope and downslope points must be at leastapproximately 100 ms apart and must not be more than approximately 350ms apart in order to confirm the mechanical event signal for use intaking event measurements. A mechanical event measurement may then betaken within an EGM-based measurement window or relative to theidentified upslope and downslope. For example maximum dP/dt may bemeasured in a window of 200 ms following a sensed or paced event.

FIG. 6 shows EGM signal 502 and RV pressure signal 508 recordingsillustrating one method for detecting a non-sustained arrhythmia atblock 218 of method 200 (FIG. 3). Pulse pressure measured in response toa preliminary arrhythmia episode detection can be used for detecting abreak in a fast rhythm, i.e. detecting a non-sustained rhythm despiteEGM-based NID criteria being satisfied.

A ventricular EGM signal 502 is shown with corresponding annotations 504indicating the RR interval length in ms and the sensed event labelcorresponding to the measured RR interval length (VS for ventricularsense intervals longer than the maximum detection interval, FS forfibrillation sense intervals falling into a fibrillation interval range,TF for fast tachycardia sense and so on). A VF counter 506 counts thenumber of VF and fast VT intervals that are sensed. The VF counter 506is not increased for ventricular sense (VS) events, which are longerthan the programmed arrhythmia detection interval ranges.

When the VF counter 506 reaches a programmed NID, a preliminary VFdetection is made at 520. For each long RR interval 516 corresponding toVS events, a corresponding mechanical event 518 is measured to determineif the long RR interval 516 is associated with normal hemodynamicfunction. Normal or near-normal hemodynamic function associated with aVS event indicates a break in the fast rhythm and is thus evidence of anon-sustained arrhythmia.

Accordingly, when a preliminary arrhythmia detection 520 is made, apreceding number of cardiac cycles are analyzed to determine if any VSevents occurred. If a VS event occurs, the hemodynamic signal 508 isanalyzed, e.g. pulse pressure is measured, during the RR intervalimmediately following the VS event to determine if the VS event isassociated with normal hemodynamic function.

In one embodiment, this analysis is achieved by implementing anon-sustained arrhythmia flag 510 and a non-sustained rule counter 512.The non-sustained rule counter 512 is initially set to a predeterminednumber of intervals, for example 16 intervals, whenever the VF counter506 initially goes from zero to one or reaches another predeterminedcount less than the NID. In the example shown, the non-sustained rulecounter 512 is initialized to 16 when the VF counter 506 reaches 4. Thenon-sustained rule counter 512 is decreased by one on each ventricularevent as long as the non-sustained flag 510 is zero.

If the VF counter 506 is not increased, i.e. if a VS event occurs, thepressure signal is measured during the RR interval immediately followingthe VS event to determine if good hemodynamic function is present inresponse to the VS interval 516. For example, the pulse pressure and themaximum dP/dt may be measured for a mechanical event 518 occurringimmediately following the VS event. The mechanical event interval 518associated with a VS event, for purposes of measuring hemodynamicfunction, may be defined relative to the VS event or based onidentifying the mechanical pressure pulse event as described previouslywherein the pressure pulse onset is searched for beginning from the VSevent. If the pulse pressure and the maximum dP/dt of the pressure pulse518 are approximately equal to or greater than the LT baselinemeasurements, the non-sustained flag 510 is set to 1. The non-sustainedrule counter 512 is reset to its initial value, as seen at 522 in FIG.6, in response to the non-sustained flag 510 being set to 1.

The arrhythmia is determined to be a non-sustained arrhythmia. The longelectrical event interval resulting in the VS event paired withhemodynamic function equaling or exceeding the LT baseline measurements,or some threshold thereof, indicates a break in the pathological rhythm.

The preliminary VF detection 520 will not be confirmed as VF until theVF counter 506 reaches the NID and the non-sustained rule counter 512reaches zero. In the example shown, the non-sustained rule counter 512is at six at 524 when the VF counter 506 reaches the NID at 520 (18 inthis example). The preliminary VF detection made in response to the NIDbeing met will not be confirmed until the non-sustained rule counter 512is zero, or until the hemodynamic analysis timer expires (as describedabove in conjunction with FIG. 3) and additional EGM analysis confirmsthe VF detection. If the non-sustained rule counter 512 reaches zero,the VF detection may be confirmed, and a VF therapy delivered as long asthe NID requirement is still being met. Thus, an arrhythmia detectionmay be withheld in response to a single VS event associated with normalhemodynamic function representing a break in the pathological rhythm.

In alternative embodiments, the VF counter 506 may be modified to bereset when the non-sustained flag 510 is high, eliminating the need fora separate non-sustained rule counter 512. The non-sustained flag ishigh whenever a VS event is immediately followed by a mechanical eventmeasurement that equals or exceeds the LT baseline measurement, oranother threshold set to represent normal hemodynamic functioncorresponding to a break in the arrhythmic rhythm.

FIG. 7 shows EGM signal 602 and RV pressure signal 604 recordingsillustrating methods for discriminating sinus tachycardia andsupraventricular tachycardia from true VT/FVT or VF using hemodynamicmeasurements. RR intervals sensed from the EGM signal 602 result in apreliminary arrhythmia detection at 606. In response to the preliminarydetection, the RV pressure signal 604 is analyzed to discriminatebetween true ventricular arrhythmias resulting in hemodynamic collapseneeding immediate therapy delivery and sinus tachycardia and SVT.

Upon the preliminary arrhythmia detection 606, the RV pressure signal isevaluated using mechanical event epochs. A mechanical event epoch isdefined as a predetermined number of mechanical event intervals, whichmay or may not be determined independently of the electrical eventintervals. An epoch may be set to include eight or more mechanical eventintervals. By including at least eight mechanical event intervals, theeffects of hemodynamic signal variation due to respiration artifact isexpected to be reduced.

However, other embodiments may define a mechanical event epoch to be anydesired number of mechanical event intervals or electrical eventintervals once the electrical event intervals have been verified to notinclude oversensing. If the electrical event intervals are verified byindependently determined mechanical events, the electrical events may beused to set time windows within which a mechanical event is confirmedand mechanical event measurements are taken, as described previously.

Mechanical event epochs are established when an arrhythmia detectioncounter reaches a predetermined number. For example, mechanical eventepochs may be established when a VT/FVT/VF or combined count counter isincreased from 0 to 1 or reaches another predetermined count, e.g., 3.

Alternatively, epochs may be established retrospectively in response toa preliminary arrhythmia detection. Epochs may be established beginningat the time of the preliminary arrhythmia detection, i.e., upon NIDcriteria being satisfied, and extend earlier in time from the arrhythmiadetection time point for a predetermined number of epochs. Accordingly,a hemodynamic signal 604 may be continuously monitored and recorded inmemory, e.g. in a looping manner, such that at any given time a minimumduration of the hemodynamic signal is available that would precede apreliminary arrhythmia detection should a detection occur. Alternativelyor additionally, epochs may be established from the time of preliminaryarrhythmia detection and extend later in time for use in detectinghemodynamic collapse during redetection of the episode.

In FIG. 7, one or more mechanical event epochs 610 through 614 is/areestablished by identifying groups of consecutive mechanical eventintervals beginning when an NID counter reaches a predetermined count.The established epochs may extend up to the time of the preliminaryarrhythmia detection 606. Each epoch 610 through 614 includes apredetermined number of mechanical event intervals, for example 8intervals. An epoch 614 may optionally extend beyond the time ofpreliminary detection 606 in order to complete the epoch with a requirednumber of mechanical event intervals. Alternatively, epoch 614 may bediscarded. In still another embodiment, epoch 614 may be accepted with afewer number of valid mechanical event intervals.

The onset of each mechanical event interval within an epoch may beidentified as described previously in conjunction with FIG. 5independently from the EGM signal or using electrical events to definewindows of time in which to search for a confirmed mechanical event andmake mechanical event measurements. Mechanical event measurements arecomputed from the mechanical events within each established epoch 610through 614. In the example shown in FIG. 7, the average pulse pressureand the average maximum dP/dt is computed for each epoch 610 through614. Three different sets of criteria may be applied to the epochmeasurements to discriminate between true VT/FVT/VF with hemodynamiccollapse, SVT and sinus VT.

In one embodiment, if the average epoch pulse pressure for the mostrecent 2 epochs (or other predetermined number of epochs) preceding thepreliminary arrhythmia detection 606 is less than a predeterminedpercentage of the LT pulse pressure baseline, hemodynamic collapse isdetected. Referring back to FIG. 3, if the mechanical intervalmeasurements confirm the electrical interval measurements at block 216,and no rhythm break is detected at block 218, the detection ofhemodynamic collapse at block 220 based on two out of two epochs havinga pulse pressure less than approximately 40% (or another predeterminedpercentage) of the LT baseline results in immediate confirmation of thedetected arrhythmia.

In addition or alternatively to the LT baseline comparison, the averagepulse pressure or another hemodynamic measure of each epoch may becompared to a predetermined fixed minimum for the detection ofhemodynamic collapse. For example, if the average epoch pulse pressureis less than approximately 10 mmHg, and the LT baseline pulse pressuremeasurement has met the qualification requirement for use in arrhythmiadetection, e.g. at least approximately 20 mmHg, hemodynamic collapse isdetected.

Upon detecting hemodynamic collapse, immediate delivery of a programmedmenu of tiered therapies is initiated. Alternatively, an override of theprogrammed menu of tiered therapies may be made in response to thehemodynamic collapse to allow immediate progression to a more aggressiveshock therapy to treat the potentially lethal arrhythmia.

If two of the three most recent epochs 610 through 614 (or otherpredetermined n out of m epochs) have an average pulse pressure andaverage maximum dP/dt greater than or equal to the ST baselinemeasurements, then the preliminary arrhythmia detection is withheld.When the ST baseline measurements are not qualified or valid, or inaddition to the ST baseline requirement, a multiple of the LT baselinemeasurement may be compared to the epoch pulse pressure and dP/dtmeasurement to allow sinus tachycardia discrimination. These criteriamay be applied at block 222 of method 200 for detecting hemodynamicevidence of sinus tachycardia. Specifically, increased pulse pressureand maximum dP/dt reaching or exceeding the ST baseline measurements,and/or a multiple of the LT baseline measurements, is evidence of sinustachycardia. Method 200 would advance from block 222 to block 232. Notherapies will be delivered until the arrhythmia detection is confirmedusing additional EGM and hemodynamic data.

If the fast ventricular rate is the result of a conductedsupraventricular tachycardia, sufficient autonomic tone is required toconduct the fast rate from the atrial chambers to the ventricularchambers. This autonomic condition is likely to be reflected in anincreased pulse pressure and increased maximum dP/dt. As such, if n outof m, e.g., two out of two (or any other predefined n out of m epochs),selected from the most recent epochs 610 through 614 preceding thepreliminary arrhythmia detection 606 have an average pulse pressure andaverage maximum dP/dt greater than or equal to the LT baselinemeasurements, and if the measured RR intervals show evidence of cyclelength instability, evidence of supraventricular tachycardia is detectedat block 224 of method 200 (FIG. 3).

A variety of methods may be used for detecting cycle length instability.For example, if two consecutive RR intervals vary by more than apredetermined amount, cycle length instability may be detected.Hemodynamic measurements may be made on the longer of the two intervalsto detect a pulse pressure and maximum dP/dt equaling or exceeding theLT baseline measurements as further evidence of an unstable cycle lengthassociated with SVT.

In evaluating hemodynamics for discrimination of SVT, sinus tachycardiaand true ventricular arrhythmias, a variable number of mechanical eventepochs may be available depending on the time from starting theestablishment of the event epochs until the preliminary arrhythmiadetection. For example, only one epoch may be available in which casethat one epoch is used to test discrimination criteria. Additionally,some epochs may be discarded because of low hemodynamic signal strength,high artifact, or other signal quality requirements are not met.Alternatively, epochs not meeting signal quality requirement may stillbe used and counted against meeting the n out of m epochs required tomeet hemodynamic requirements for withholding an arrhythmia detectionand/or detecting hemodynamic collapse. It is recognized that thespecific thresholds of n out of m epochs meeting various hemodynamiccriteria may vary between embodiments and may be programmableparameters.

FIG. 8 is a state diagram illustrating the transitions between apreliminary detection state 805, a confirm detection state 810 and awithhold detection state 815. Initially, LT and ST baseline measurementsare obtained and qualified at state 801. An arrhythmia detection andtherapy delivery decision algorithm is then enabled. The preliminarydetection state 805 is initially entered whenever electrical eventintervals meet NID criteria 802. The preliminary detection state 805moves to the confirm detection state 810 in response to a hemodynamiccollapse rule 804 being satisfied or an EGM morphology analysis rule 806being satisfied and thus confirming the detected arrhythmia

In one embodiment the hemodynamic collapse rule 804 requires:

-   -   a) RR intervals meet NID criteria,    -   b) mechanical event intervals confirm electrical event        intervals,    -   c) rhythm is sustained (no break detected), and    -   d) hemodynamic collapse detected (e.g., n out of m most recent        epochs have average pulse pressure less than a percentage of the        LT pulse pressure).

Therapy is delivered in the confirm detection state 810 according to aprogrammed menu of therapies or according to a menu override response tothe detected hemodynamic collapse.

Transition from preliminary detection state 805 directly to confirmdetection state 810 may also occur when EGM morphology analysis 806 isperformed in response to no hemodynamic collapse being detected and nohemodynamic evidence of sinus tachycardia or SVT being detected. Inother words, the hemodynamic analysis may fall into a “gray area” inwhich none of the hemodynamic discrimination rules are met (relating todetecting hemodynamic collapse, sinus tachycardia, or SVT). When thehemodynamic measures do not meet rules for withholding the arrhythmiadetection, i.e. sinus tachycardia or SVT rules are not met (as describedfurther below), and hemodynamic collapse is not detected to confirm thearrhythmia, the EGM morphology analysis is performed in an attempt toconfirm the preliminary detection. The EGM morphology rule 806 requiresall of the following criteria to be met in one embodiment:

-   -   a) RR intervals meet NID criteria,    -   b) mechanical event intervals confirm electrical event        intervals,    -   c) rhythm is sustained (no break detected),    -   d) sinus tachycardia and SVT rules not met (and no hemodynamic        collapse), and    -   e) EGM morphology analysis results in confirmed arrhythmia        morphology detection.

The preliminary detection state 805 transitions to a withhold detectionstate 815 in response to any of the following rules: rate not confirmed811, non-sustained arrhythmia 812, sinus tachycardia evidence 814, orSVT evidence 816 being met. Though not explicitly shown in FIG. 8, anadditional EGM morphology based rule may cause transition from state 805to withhold detection state 815. In particular, when none of the rules811 through 816 are satisfied, and hemodynamic collapse is not detected,EGM morphology analysis may cause the arrhythmia detection to bewithheld.

In one embodiment, the rate not confirmed rule 811 requires:

-   -   a) average pulse pressure greater than or equal to the LT        baseline pulse pressure    -   b) average pulse pressure greater than or equal to a        predetermined minimum signal strength threshold (e.g., 20 mmHg),        and    -   c) mechanical event interval length greater than approximately        1.8 times the electrical event interval length.

When the rate not confirmed rule 811 is satisfied, T-wave oversensingmay be occurring. Additional EGM signal monitoring is performed.

The non-sustained arrhythmia rule 812 requires:

-   -   a) a VS event occurring when the VF count is greater than zero,    -   b) a pulse pressure immediately following the VS event is        greater than or equal to the LT baseline pulse pressure,    -   c) a pulse pressure immediately following the VS event is        greater than or equal to a minimum signal strength threshold,        and    -   d) a maximum dP/dt immediately following the VS event is greater        than or equal to the LT baseline dP/dt.

If the non-sustained arrhythmia rule 812 is satisfied, a break in thefast rhythm associated with good hemodynamic function is present. Thearrhythmia detection is withheld.

The sinus tachycardia rule 814 requires:

-   -   a) n out of m mechanical event epochs have an average pulse        pressure greater than the minimum signal strength threshold,    -   b) n out of m epochs have an average pulse pressure greater than        or equal to the ST baseline pulse pressure and greater than or        equal to a multiple of the LT baseline pulse pressure (or only a        multiple of the LT baseline pulse pressure if valid ST baseline        is not available), and    -   c) n out of m epochs have an average dP/dt greater than or equal        to the ST baseline dP/dt and greater than or equal to a multiple        of the LT baseline dP/dt (or only a multiple of the LT baseline        dP/dt if valid ST baseline is not available).

When the sinus tachycardia evidence rule 814 fires, evidence of sinustachycardia based on increased hemodynamic function causes thearrhythmia detection to be withheld.

The SVT rule 816 requires:

-   -   a) RR interval instability criteria met,    -   b) a longer RR interval consecutively following a shorter RR        interval having a pulse pressure greater than or equal to a        minimum signal strength threshold and greater than or equal to        the LT baseline pulse pressure,    -   c) the longer RR interval consecutively following the shorter RR        interval having a maximum dP/dt greater than or equal to the LT        baseline dP/dt,    -   d) n out of n epochs have an average pulse pressure greater than        or equal to minimum signal strength threshold and greater than        or equal to the LT baseline pulse pressure, and    -   e) n out of n epochs have an average maximum dP/dt greater than        or equal to the LT baseline pulse pressure.

When the SVT rule 816 fires, evidence of RR interval instability andnormal hemodynamic function suggests the fast ventricular rate detectedusing the EGM signal is caused by an SVT. Accordingly, the arrhythmiadetection is withheld in state 815.

The withhold detection state 815 transitions back to the preliminarydetect/redetect state 805 when RR intervals meet NID redetectioncriteria 818. The withhold detection state 815 may also transition backto state 805 if a hemodynamic analysis timer expires 820 and EGMmorphology analysis performed in response to timer expiration does notconfirm the rate-based detection.

Upon returning to state 805 in a redetection condition, some of thehemodynamic rules may be disabled for the remainder of the detectedepisode. For example, once the mechanical event intervals confirm theelectrical event intervals in response to a preliminary arrhythmiadetection, the mechanical event interval rule may be disabled thereaftersuch that mechanical event intervals cannot later fail to confirm theelectrical event intervals during the same preliminarilydetected/redetected episode. In another embodiment, the hemodynamiccollapse rule and/or its associated therapy sequencing may only beapplied during a redetection not following a preliminary detection.Alternatively or additionally, the hemodynamic collapse rule may beapplied only when the preliminary detection/redetection is FVT or VF,not VT.

The withhold detection state 815 may transition to the confirm detectionstate 810 if EGM morphology analysis 822, performed in response toexpiration of a hemodynamic timer, confirms the arrhythmia detection.Therapy is delivered according to a programmed menu of therapies.

Thus, an implantable medical device and associated methods forarrhythmia detection and therapy delivery decisions have been presentedin the foregoing description with reference to specific embodiments. Itis appreciated that various modifications to the referenced embodimentsmay be made without departing from the scope of the invention as setforth in the following claims.

1. A method for detecting an arrhythmia, comprising; sensing cardiacelectrical signals; detecting from the sensed electrical signals anepisode of cardiac electrical event intervals meeting cardiac arrhythmiadetection criteria; detecting one of the cardiac electrical eventintervals of the detected episode as a long interval not meeting anarrhythmia interval criteria; sensing a hemodynamic signal; determininga first cardiac mechanical event measurement from the hemodynamic signalimmediately following the long interval; and withholding a cardiacarrhythmia detection of the episode in response to the first cardiacmechanical event measurement.
 2. The method of claim 1 furthercomprising: computing a first baseline mechanical event measurement fromthe hemodynamic signal; and withholding the cardiac arrhythmia detectionin response to the measured cardiac mechanical event being at leastapproximately equal to the first baseline.
 3. The method of claim 2wherein the first baseline is computed over at least a twenty-four hourperiod.
 4. The method of claim 2 further comprising: detecting aninterval of increased metabolic demand prior to the episode; computing asecond baseline mechanical event measurement from the hemodynamic signalsampled during the interval of increased metabolic demand; identifyingfrom the hemodynamic signal a plurality of mechanical event intervalsduring the episode; computing a second mechanical event measurement fromthe plurality of mechanical event intervals; comparing the secondmechanical event measurement to the second baseline measurement; andwithholding an arrhythmia detection of the episode in response to thesecond mechanical event measurement exceeding the second baselinemeasurement.
 5. The method of claim 4 wherein detecting an interval ofincreased metabolic demand comprises detecting a heart rate of at leastapproximately 90 beats per minute.
 6. The method of claim 2 furthercomprising: computing a measurement of a plurality of cardiac mechanicalevents occurring during the episode; and withholding the cardiacarrhythmia detection in response to the measurement of the plurality ofcardiac mechanical events being at least approximately equal to thefirst baseline.
 7. The method of claim 1 further comprising: identifyingfrom the hemodynamic signal a plurality of mechanical event intervalsduring the episode; computing a mechanical event interval measurementfrom the plurality of mechanical event intervals; computing from theelectrical signal an electrical event interval measurement during theepisode; comparing the mechanical event interval measurement to theelectrical event interval measurement; and withholding the cardiacarrhythmia detection of the episode in response to the comparing.
 8. Themethod of claim 7 wherein the cardiac arrhythmia detection is withheldif the mechanical event interval measurement is greater thanapproximately 1.8 times the electrical event interval measurement. 9.The method of claim 7 further comprising: computing a first baselinemechanical event measurement from the hemodynamic signal; computing ameasurement of a plurality of cardiac mechanical events occurring duringthe episode, detecting hemodynamic collapse in response to themeasurement of the plurality of cardiac mechanical events being lessthan a percentage of the first baseline; and confirming a cardiacarrhythmia detection of the episode in response to detecting hemodynamiccollapse.
 10. The method of claim 9 further comprising overriding aprogrammed menu of arrhythmia therapies in response to the detectedhemodynamic collapse.
 11. An implantable medical device for detecting anarrhythmia, comprising; a sensing module sensing cardiac electricalsignals and a hemodynamic signal; and a control module coupled with thesensing module and detecting from the sensed electrical signals anepisode of cardiac electrical event intervals meeting cardiac arrhythmiadetection criteria, detecting one of the cardiac electrical eventintervals of the detected episode as a long interval not meeting anarrhythmia interval criteria, determining a first cardiac mechanicalevent measurement from the hemodynamic signal immediately following thelong interval, and withholding a cardiac arrhythmia detection of theepisode in response to the first cardiac mechanical event measurement.12. The device of claim 12 wherein the control module computes a firstbaseline mechanical event measurement from the hemodynamic signal, andwithholds the cardiac arrhythmia detection in response to the firstcardiac mechanical event measurement being at least approximately equalto the first baseline.
 13. The device of claim 12 wherein the firstbaseline is computed over at least a twenty-four hour period.
 14. Thedevice of claim 12 wherein the control module: detects an interval ofincreased metabolic demand prior to the episode, computes a secondbaseline mechanical event measurement from the hemodynamic signalsampled during the interval of increased metabolic demand; identifiesfrom the hemodynamic signal a plurality of mechanical event intervalsduring the episode; computes a second mechanical event measurement fromthe plurality of mechanical event intervals; compares the secondmechanical event measurement to the second baseline measurement; andwithholds an arrhythmia detection of the episode in response to thesecond mechanical event measurement exceeding the second baselinemeasurement.
 15. The device of claim 14 wherein detecting an interval ofincreased metabolic demand comprises detecting a heart rate of at leastapproximately 90 beats per minute.
 16. The device of claim 12 whereinthe control module: computes a measurement of a plurality of cardiacmechanical events occurring during the episode; and withholds thecardiac arrhythmia detection in response to the measurement of theplurality of cardiac mechanical events being at least approximatelyequal to the first baseline.
 17. The device of claim 12 wherein thecontrol module: identifies from the hemodynamic signal a plurality ofmechanical event intervals during the episode; computes a mechanicalevent interval measurement from the plurality of mechanical eventintervals; computes from the electrical signal an electrical eventinterval measurement during the episode; compares the mechanical eventinterval measurement to the electrical event interval measurement; andwithholds the cardiac arrhythmia detection of the episode in response tothe comparison.
 18. The device of claim 17 wherein the cardiacarrhythmia detection is withheld if the mechanical event intervalmeasurement is greater than approximately 1.8 times the electrical eventinterval measurement.
 19. The device of claim 17 wherein the controlmodule: computes a first baseline mechanical event measurement from thehemodynamic signal; computes a measurement of a plurality of cardiacmechanical events occurring during the episode, detects hemodynamiccollapse in response to the measurement of the plurality of cardiacmechanical events being less than a percentage of the first baseline;and confirms a cardiac arrhythmia detection of the episode in responseto detecting hemodynamic collapse.
 20. The device of claim 19 furthercomprising: a therapy delivery module; and a therapy delivery controlcoupled to the therapy delivery module for delivering a menu of tieredtherapies in response to the confirmed cardiac arrhythmia detection;wherein the control module overrides the programmed menu of arrhythmiatherapies in response to the detected hemodynamic collapse to cause thetherapy delivery module to delivery a shock therapy.